area irreproducibility,

Discussions about GC and other "gas phase" separation techniques.

13 posts Page 1 of 1
I have problem with area irreproducibility sometimes I obtain area 117 next inject and I have area 88 I don't understand it
sample: 0,1 g methanol in 100ml ethanol
column : HP innowax 15 m 0,25 mm film ticknes 1 micrometer
flow: 3
inlet preassure 1,2 psi
split 30:1
inlet temp: 250 dC
detect temp: 250 dC
manual inject: 1 microlitr
I want to say that I use three difrent syringe I don't have leek in my equipment I change septa and ofcourse liner. I tray to decrising inlet temperature but without possitive effect .

Assuming you have proper id of peaks and consistent retention Id suggest two fairly quick troubleshooting options.
#1 Try adding an internal standard and determining the relative area. This will let you know if your injections are being affected uniformly or if its peak/column related
#2 Kill the split flow if it wont overload your column, to see if your just merely blowing off your peak.

are you exceeding the headspace volume of your liner? maybe a decrease in inj volume would help also...but at 1mic i dont think this is the problem...
You can also try baking the column for a little while to clean it out

Good luck and keep us posted
regards,
~law_son
I would suspect your injection technique is the most likely problem assuming you have no leaks and the column is installed correctly.

the septum must not be leaking and the residence time of the needle in the injector must be consistent. The amount in the needle must also be consistent.

If you have no leaks and the vaporization cloud of the injection is reproducible to the position of the column inlet you should have reproducible areas.

I have performed manual injections with less than a 1% RSD but it ain't easy and requires very good technique.

best wishes,

chromatographer1

I try adding an internal standard and determining the relative area:
standard 500 ppm aceton
I obtain difrent volues of areas both methanol and aceton
Now I have another problem my set preassure don't reached to the actual preassure But only in split below 25:1

As chromatogrpher1 first suggested it looks like injection technique is the likely issue. But your split valve could also be the problem. Try an injection using no split flow, maybe lowering the concentration of the sample or decreasing the inj amt. If after that your area's are still inconsistent you may want to consider finding an autoinjector.

Best of luck,

~law_son

additional considerations than those already proposed:

ensure that none of the syringes leak; if syringes are not identical, use one syringe for all the injections

with respect to the pressure problems, calibrate the flow, including the septum purge flow. if there are no internal gas leaks as you indicate, ensure there are no external leaks (between gas tanks and gc).

i suggest making 5-10 sequential injections to define the data dispersion in your methodology, since you only indicated a couple of runs (unless this was the range of data scatter). you might also want to compare data on different days for similar injections. also is the irreproducibility random (normal data dispersion) or is there a trend (such as the peak areas tend to decrease with number of sequential injections)?

good luck.

I'm not surprised that you are having reproducibility problems. You are injecting a solvent with a relative high expansion volume at a very low head pressure. Also, in my experience the Agilent GC does not do a very good job at controlling head pressure at settings below about 10 psi, which is not surprising since you are at the very bottom of the range of the flow controller. Also, is is hard to control the pressure at a low value with very much flow going out the split vent. It is hard to have a low pressure with a high flow. With the conditions listed I am surprised the GC will come ready.

I would suggest checking all the settings, as a head pressure of 1.2 psi should not give a flow of 3 mL/min with a 0.25 id column. You might also want to consider the use of a longer column to allow a higher head pressure to reduce the expansion volume. Finally, I would use a column that would allow a split of at least 100:1 for a sample like this to reduce column overloading and improve peak shape.

One thing I neglected to mention is that the peak areas of analytes eluting before the solvent peak are often inconsistent at low split ratios. Higher split ratio should help.

Thanks a lot Ron and other helpfull Guest.
I think Ron that you have right with this bad job of Agilent GC
I cant do what you say because when I decrease split ratio more than 35:1, setpoint preassure(1,9psi) is lower than actual preassure(1,7 psi) but when I change column on PEG film thicknes 0,25 mirometer id 0,25 mm lenght 30 m I can make split for example 100:1 but my preassure is greter than 20 psi and actual and setpoint pressure is constant.
Ofcourse other guest have right that my technique of injection was bad but I want to correct my tehnique.Now my standard deviation is steel too high 10 - 15 %

:D

What do you use for liner :?:

You should try a liner with constriction (if it is not the case) to prevent the band of liquid exhausting the syringe to being shot at the bottom of the liner.

Like a laminar liner,,carbofrit (best for Polar),or a liner with glass wool in the middle,that should improve the evaporation process in the injector.

Your system seems in order now, and your still high rsd%,is mainly related to a poor (not reproductible) evaporation process.

By

Thanks xxx
but I use a good liner as I think! I use liner with a glass in the midle of it

:D

With Glass Wool in the middle you mean :P .

Yes that it's right
Sorry for my english :oops:
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